This invention relates generally to methods, sensors and instrumentation for measuring the strength of electromagnetic fields and power, particularly in the microwave region of the spectrum.
The use of microwave energy for heating dielectric materials rapidly is well-known and increasing. The majority of homes in the United States now have microwave ovens. As a result, a wide variety of new microwave food products, formulated and packaged for most effective "cooking" in the microwave oven, are being developed and introduced to the market.
At the same time, electromagnetic radiation within the microwave and radio frequency ranges is being used increasingly in industry for processing various dielectric materials--e.g., plastics, rubber, wood products and ceramics. The primary desired result is to produce throughout the product volume very rapid heating and thereby quick drying, curing, firing and so on. The primary variable under the control of the user or operator is the average applied power which in turn relates directly to the average strength of the fields present in the processing chamber.
The incident electromagnetic waves in these and similar processes consist of time-varying E (electric) and H (magnetic) fields oriented at right angles to one another and to the direction of propagation. Either the E-field or H-field can produce heating but normally the E-field produces the dominant effects. The E-field produces oscillating current flow and thereby resistive heating if the medium is conductive. The E-field can also produce oscillating motion of bound charges (polarization) and reorientation of permanently polar molecules (such as water), and these effects also lead ultimately to heating. Interest in H-field measurements is limited primarily to the processing of magnetic materials or, occasionally, to the indirect measurement of currents induced by electromagnetic waves in processing chamber walls. The power available at a given point in a processing chamber is proportional to the square of the effective field, either E or H.
There are several reasons for measuring these fields, particularly the E-field, either prior to or during processing of materials with electromagnetic energy. First, knowing the field variations (or the average power distributions derived therefrom) in the processing chamber allows an improved prediction of heating rates and uniformity. If an altered power distribution is desired, the chamber or the product can be modified as necessary.
Second, during processing, the material being processed will further alter the microwave field and power distributions within the material because of the larger dielectric constant of the material relative to free space or air and the absorption of microwaves by the material. Knowledge of these effects can be further utilized to optimize the size, composition, and shape of the product and/or the speed of processing.
Third, because of the absorption of microwave energy by the material being processed, the fields and power at other points in the chamber will be modified (typically reduced). Thus, by measuring the fields or power at a point outside of the material, it may then be possible to monitor and control its processing. Such a remote sensor can either be located elsewhere within the processing chamber or in a waveguide leading from the chamber to a dummy load.
Fourth, a particular problem of interest involves optimizing the design of a microwave food product utilizing "active" packaging--i.e. packaging which either preferentially concentrates or homogenizes the microwave energy distribution incident on the food product or, in the case of metallic susceptor films, absorbs additional microwave energy locally in the film to provide preferential heating of adjacent portions of the food product (e.g., the crust of a pizza or sausage roll). A very small probe capable of measuring local fields within the package or food product would be useful in evaluating the degree to which the microwave power distribution internal to the product has been altered by the specific package design.
A fifth reason for measuring the level of such fields pertains to computer modeling. As better computer models are developed for analyzing and eventually optimizing food product designs, good input data, including local microwave fields, will be needed for these models. Field probes will therefore be needed to provide this information internal to the product.
Present field measurement techniques are electrical and utilize electrically-conducting antennas of various designs. These antennas have typically been developed for measuring weak fields, as for example the leakage of microwaves from a closed, operating oven. Such antennas cannot survive, let alone measure, the intense heating fields inside of an operating oven or processing chamber.
Such leakage field monitoring antennas have been designed for the most efficient coupling to the fields being measured. For maximum efficiency of coupling, a dipole antenna is typically designed with a length that is one-half the wavelength of the radiation being measured. The microwave oven operates at 2450 MHz. The wavelength corresponding to this frequency in free space (or air) is about 12 cm. Hence, a half wavelength dipole is a little over 2 inches in length. An orthogonal array of three crossed dipoles is normally used to provide orientation-independent measurements. This array is typically contained in a polystyrene foam sphere, both for protection and for insulation. The outer sphere then has a diameter of about 21/2" or greater, a size which is too large to allow detailed mapping within the oven and which would be impossible to use for measurements internal to a typical food product.
It is preferable not to use highly conductive metals within the regions being measured since the presence of such conductors alter the fields being measured. Both the electrical antenna and its leads would perturb the measurement environment, leading to incorrect values for the field distributions. In addition, the presence of these conductors may also lead to electrical arcing and shorting to ground, again interfering with the measurement.
For these reasons, it is suggested instead that the field measurements be made by thermal techniques, using a small, minimally-perturbing sensor designed to be heated predictably by the oscillating fields with the resultant temperature rise then being measured by passive fiberoptic techniques. For example, Luxtron Corporation, assignee of the present application, has perfected a phosphor-based temperature sensing technology which can be used with either remote viewing or with the phosphor sensor attached to the end of an optical fiber. By monitoring the fluorescent decay time of the phosphor, one can determine its temperature. (The application of such a technique for measurement of high frequency fields and power in conjunction with a resistive antenna of conventional dimensions relative to the wavelengths of interest has been suggested by Martin et al. "Fiber Optic Microwave Power Probe", Optical Engineering. Feb. 1987, vol. 26, no. 2, pp. 170-173, and by Randa et al, "High Frequency Electric Field Probe Development". Symposium Record of the International Conference on Electromagnetic Compatibility, May 10-12, 1988.)
Recently this luminescent fiberoptic technique has been applied to the measurement of currents induced by high frequency fields in the firing circuits of electroexplosive devices ("EEDs") used in ordnance systems. The bridge wire of the EED is a resistive conductor whose current is raised by the high frequency induced current flowing through it. As expected, the temperature rise is proportional to the resistance and to the square of the induced current. The EED thus becomes a current sensor for detecting currents induced in the cabling of the firing circuit connected to it. Calibration of the sensor is performed by injecting known currents and noting the resultant temperature rises. Such an EED technique is described by Gibes et al., "An Improved Fiber Optic Current Sensing System for High Frequency RF Susceptibility Measurements", EMC Technology, Nov.-Dec. 1987, vol. 6, no. 7, pp. 45-50.
It is a primary object of the present invention to provide sensors, instrumentation and techniques for measuring high levels of electromagnetic power, in microwave and other regions, with a sensor that is physically quite small, does not disturb the thermal characteristics or the electromagnetic fields of the environment being measured, which provides an accurate, repeatable measurement of the field, and which is easy and economical to implement.